An LCD unit includes a thin-film-transistor (TFT) substrate and a counter substrate sandwiching therebetween a liquid crystal (LC) layer. The TFT substrate mounts thereon an array of TFTs corresponding to an array of pixels in the LCD unit. The TFTs are driven separately from one another via a plurality of gate lines and a plurality of drain lines (data lines) by using a drive LSI. The each TFT functions as a switching device to control the electric field applied to the LC layer, in a pixel-by-pixel basis, for displaying an image on the screen of the LCD unit.
The TFT substrate is a transparent substrate or a glass substrate, on which the array of TFTs are formed. Each TFT includes a gate electrode formed on the glass substrate, an intrinsic semiconductor layer formed on the gate electrode with an intervention of a gate insulating film, and source and drain electrodes electrically connected to the intrinsic semiconductor layer on both ends of the channel region thereof. The gate electrode is configured as a part of a gate electrode layer, whereas the source and drain electrodes are configured as a part of a source/drain electrode layer. The intrinsic semiconductor layer is configured by a hydrogenated amorphous silicon (a-Si:H) etc.
The structure of the TFT substrate in the LCD unit is described in, for example, the following publications:
Patent Publication 1; JP-2000-171834A; and
Patent Publication 2; JP-1999-2843A.
Along with the recent development of a higher performance of the LCD unit, it is desired that the TFT has a higher ON/OFF ratio of the drain current in order to improve the data writing characteristic of the TFT. The higher ON/OFF ratio of the drain current may be obtained by a smaller thickness of the gate insulating film to thereby increase the ON current of the TFT.
However, there is a tendency that a thickness of the gate insulating film is too thin, under a specific thickness of the film, to cause a significant increase in the OFF current of the TFT and which may incur a poor image quality, such as non-uniformity of the screen image and point defects on the screen of the LCD unit. The thickness of the gate insulating film has reached almost the lower limit of thickness, the OFF current significantly increases and as a result, the ON/OFF ratio of the drain current goes down. Thus, it is difficult to further reduce the thickness of the gate insulating film without increasing the OFF current of the TFT.